The most commonly used anticancer drugs are more ctyotoxic toward normally oxygenated tumor cells than toward hypoxic tumor cells. Hypoxic cell resistance to irradiation is also widely known. Consequently, tumor hypoxia and the resultant resistance to treatment is of concern in cancer therapeutics.
Solid cancer tumors contain both adequately oxygenated cells as well as varying proportions of inadequately oxygenated or hypoxic cells. Hypoxia usually occurs where the tumor cells are furthest away from blood vessels. Such cells also tend to have slower rates of proliferation. Although not completely understood, resistance of hypoxic cells to anticancer drugs is generally thought to be due to inadequate uptake of the drug by the hypoxic cells either because they tend to be slowly growing or because of their distance from the blood vessels bringing the drug. Thus, the relative proportion of hypoxic cells in the tumor can be of great importance to the outcome of the treatment. Resistant hypoxic cells that survive irradiation or drug treatment may become reoxygenated, thereby restoring tumor sensitivity to further treatment. Nonetheless, instead of relying on uncertain events, it is desirable to develop cancer treatments wherein cancer tumor cells, including hypoxic tumor cells, are killed or rendered inactive more reliably at the time the treatment is administered.
U.S. Pat. No. 5,175,287 issued Dec. 29, 1992 discloses the use of 1,2,4-benzotriazine oxides in conjunction with radiation for treatment of tumors. The 1,2,4-benzotriazine oxides sensitize the tumor cells to radiation and make them more amenable to this treatment modality.
Holden et al (1992) "Enhancement of Alkylating Agent Activity by SR-4233 in the FSaIIC Murine Fibrosarcoma" JNCI 84: 187-193 discloses the use of SR-4233, also known as tirapazamine, in combination with an antitumor alkylating agent. The four antitumor alkylating agents, cisplatin, cyclophosphamide, carmustine and melphalan, were each tested to examine the ability of tirapazamine to overcome the resistance of hypoxic tumor cells to antitumor alkylating agents. Tirapazamine was tested alone and in combination with varying amounts of each of the antitumor alkylating agents. When SR 4233 was administered just before single-dose treatment with cyclophosphamide, carmustine or melphalan marked dose enhancement leading to synergistic cytotoxic effects on tumor cells was observed. When SR 4233 was administered just prior to single-dose treatment with cisplatin, however, the dose enhancement lead to an additive effect, except at the highest dose level of cisplatin.
Nitroimidazole hypoxic cytotoxic agents have been combined with various anticancer drugs and it was found that a therapeutic gain could be achieved when these agents were combined with various anti-cancer drugs, particularly the alkylating agents, cyclophosphamide and melphalan and the nitrosoureas, BCNU and CCNU. However, it was later found that the therapeutic gain produced was not the consequence of selective killing of hypoxic cells by the nitroimidazoles but appeared to be by a mechanism involving the potentiation of alkylating agent-induced DNA cross-links by metabolites of the nitroimidazoles (Murray et al. (1983) Br. J. Cancer 47: 195-203).